Ionization equilibria of dicarboxylic acids in dimethyl sulfoxide as studied by NMR

The ionization equilibria of dicarboxylic acids measured by K 1 / K 2 are expected to be substantially influenced by stabilization of the monoanion through intramolecular hydrogen bonding. However, except in sterically favorable cases, such as with maleic, 3,4‐furandicarboxylic, 2,3‐bicyclo[1.2.2]heptadienedicarboxylic and diethylmalonic acids, the K 1 / K 2 values for other dicarboxylic acids, where intramolecular hydrogen bonding of the monoanion might be expected, are relatively small at ∼100 in water. This research uses NMR to corroborate and extend the pioneering conductance measurements of K 1 / K 2 of dicarboxylic acids by Kolthoff and coworkers with dimethyl sulfoxide (DMSO) as solvent. In extending these measurements, we have determined K 1 / K 2 for 3,4‐furandicarboxylic ( 1 ) and 2,3‐bicyclo[1.2.2]heptadienedicarboxylic ( 2 ) acids, because these were predicted by McCoy to be especially sterically favorable for intramolecular hydrogen bonding in the form of their monoanions and, indeed, we have found K 1 / K 2 values for these acids in DMSO of 1 × 10 15 and 4 × 10 16 respectively. In contrast, the corresponding value for succinic acid is about 10 8 . Copyright © 2002 John Wiley & Sons, Ltd.